Download an IRS 1040 Form.
Assuming you use the cash method (versus the accrual method) of accounting, determine his filing status and exemptions based on what they would have been. Refer to IRS Publication 559 for more information about accounting methods.
Fill out the Form 1040. Determine his total income received prior to his death. This can include his salary, tips, fringe benefits, alimony, welfare benefits, unemployment benefits, dividends and more that were received prior to death. Refer to IRS Publication 525 for a complete list of taxable income. Subtract his exemptions, deductions and adjustments from his total income to get his taxable income. Then refer to the IRS tax tables to determine his tax liability. If he qualified for any tax credits, deduct them from his liability.
If he is due a refund, and you are not the spouse or court-appointed representative, you must attach a Form 1310, Statement of Person Claiming Refund Due a Deceased Taxpayer. Representatives will need to attach a copy of the court certificate to the return.
Write "DECEASED," the name of the decedent and date of death across the top of the return. Write the decedent's name and surviving spouse's name and address in the name and address section if filing a joint return. Write the decedent's name and the personal representative's name and address in the name and address section if not filing a joint return.
Sign the return. Surviving spouses without a personal representative must add "filing as a surviving spouse."
Determine if you need to file a Form 1041, U.S. Income Tax Return for Estates and Trusts or Form 706, United States Estate (and Generation Skipping Transfer) Tax Return.